Paging systems are known whereby a centrally located operator can call up a person by name over a tannoy system.
Radio paging systems are also known whereby in a similar manner, but with greater secrecy, each one of a group of people carrying individual radio receivers can be called up using a radio transmitter to transmit an appropriate call up signal to cause the appropriate one of the radio receivers to emit a signal (either audible or visual or both) to thereby alert the carrier to make contact with the central operator.
Both systems rely on a telephone system within the environment within which the called up people are located to enable the paged person to make contact with the central operator.
Neither system however enables the operator to determine in advance whether any particular carrier is actually within the environment and where, in the environment, the carrier is actually located at any particular time. In each case the operator only becomes aware of whether the carrier is within the environment let alone where the carrier is located, after the carrier has made contact with the operator by establishing a telephone link using a locally situated telephone receiver.
U.S. Pat. No. 4,601,064 to Shipley describes a communication system for locating or tracking persons or objects to automatically establish two-way conversation with the person located via an associated intercom or telephone system. The locating system requires each person or object to be located, to carry an infra-red pulse transmitter which operates continuously automatically and repetitively and generates a unique pulse train so that each carrier can be identified by decoding its pulse train. In different regions are located infra-red receiver units which are set to continuously receive transmitter pulse signals. Each receiver includes a memory capable of storing the identifying data (derived from a received pulse transmitter) of one carrier, and to avoid confusion the identifying data stored in the receiver is erased either as a result of the receiver being polled or by the reception of a new pulse train, whether from the same or a different transmitter.
Such a system is economically as regards memory capacity for each receiver but clearly data can be lost if the polling rate is not high enough.
In order to permit a high polling rate Shipley proposes a 20 wire multiple cable to enable a parallel data bus system to be used which enables a very high polling rate, so that each receiver should be interrogated at least as frequently as randomly transmitted pulse transmissions will be received from carriers in the region.
In order to reduce the need for such high polling rates, Shipley proposes that if lower polling rates are used, certain identifying pulse train data signals may be lost due to being replaced by a later transmitted pulse train from another transmitter. On a statistical basis, if one of the identifying data signals is not seen during the end of one interval by a first polling, statistically it may be resident in the receiver memory during a subsequent polling and Shipley therefore proposes to accumulate values over a number of polls in order to identify a number of transmitters in a given region, but only after a number of pollings have occurred. This would certainly enable a number of different transmitter units within one region to be separately seen by the receiver unit associate therewith and recorded by the central processor but not as a result of a single polling, only after a number of pollings have occurred.
In the experience of the inventors of the present invention, however, systems such as Shipley are high in cost in view of the vast amount of copper cable which is required and such systems are also unreliable due to the large number of interconnections and the possibility of high resistance or open circuit connections occurring and the resulting loss of data. In addition, the electrical capacitance of such cabling is such that if the length of multiway cable is excessive, the bandwidth limitation can easily render the system incapable of operating at the polling rates required by the Shipley system so that essentially such a system can only be considered for a relatively small building.
The invention of the present application has no such problems, and obviates the inherent design disadvantages incorporated in the Shipley system, that is, the need for a parallel data bus, high polling rates and multiple polling.
It is therefore an object of the present invention to provide a system which does not require a parallel data bus nor high polling rates nor multiple pollings by which a central controller (which may be a person or programmed computer), can rapidly determine whether a particular carrier is located within a given environment and if so where the carrier is located within the environment, to thereby enable the controller to make contact with the carrier by using a standard internal telephone system to cause a telephone near to where the carrier is locate to ring, and route an incoming telephone call directly to a telephone convenient to where the carrier is actually located at the time the call is received.